Recently, various kinds of cryogenic equipments are used. A representative one is superconductive magnet mainly used in MRI. In the superconductive magnet shown in FIG. 1, a coolant vessel 2 is located in vacuum vessel 1. Liquid helium 3 as coolant is taken in the coolant vessel 2. A superconductive coil 4 is located in the liquid helium 3 to cool the coil as dunk cooling method. However, in this method, the liquid helium is necessary to be supplied in the coolant vessel in case the liquid helium is evaporated. In general, a thermal shield plate 5 is set as surrounding the coolant vessel 2 and cooled by a refrigerator 6. In order to suppress the evaporation of the liquid helium 3, heat leakage is absorbed by radiation of the thermal shield plate 5. In this method, interval of supply of the liquid helium 3 becomes long, but the supply of the liquid helium is also necessary.
As another method shown in FIG. 2, the superconductive coil 4 is directly cooled by a cryogenic refrigerator 7 without the liquid helium. This method is realized by a reason that the cryogenic refrigerator 7 is greatly developed. For example, small size refrigerator such as GM (Giford Macmaphone) can cools the coil till temperature of the liquid helium. In this superconductive magnet of conductive cooling, the liquid helium is not necessary to be supplied, construction of apparatus is simple and cost becomes low. FIG. 3 shows another example of superconductive magnet of conductive cooling. In FIG. 3, GM refrigerator of two-stage expansion method is used as cryogenic refrigerator 7. The thermal shield plate 5 is cooled to 70 K by the first cooling stage 8 and the superconductive coil 4 is cooled to 4 K by the second cooling stage 9. Furthermore, heat conduction member 10 thermally connects the second cooling stage 9 and the superconductive coil 4. In this construction, size of the superconductive magnet becomes to be one third in comparison with that of dunk cooling method. However, in this method, vibration occurred by the cryogenic refrigerator 7 is conveyed to the superconductive coil 4 and it takes a long time to cool from a normal temperature to a fixed temperature. Furthermore, minitualization of all of the apparatus has a limit because the cryogenic refrigerator 7 is necessary to be used.
As a new cooling method to solve these problems, present inventors made a proposal of cool accumulation method as shown in FIG. 4(Japanese Patent Application PH8-61458). In this method, cooling apparatus is divided into a cooling unit 16 of the cryogenic refrigerator 7 and a cold reserved unit 12 of the vacuum vessel 1 to store the superconductive coil 4. While the superconductive coil 4 is cooled till superconductive transition temperature and transferred to persistent current mode, the superconductive coil 4 and the thermal shield plate 5 are cooled by thermally connecting to the cooling unit 11 through heat conduction members 13, 14. When cooling is completed, the cooling unit 11 is separated from the cold reserved unit 12 to be used by itself. As the heat conductive members 13, 14, thermal connection method through expansion wall without vacuum break of the cooling unit 11 and the cold reserved unit 11, or thermal connection method by combination of the expansion wall and vacuum value is considered. In this method, vibration of the refrigerator does not occur and electric source is not necessary because the cooling unit 11 is separated from the cold reserved unit 12. Furthermore, one cooling unit 11 is commonly used for a plurality of cold reserved unit 12. All of apparatus is minitualized because only the cold reserved unit 12 is set to be used in actual spot.
As mentioned-above, the cool accumulation method has lots of merits. However, it is a problem that cold reserved time (adiabatic time) of the cold reserved unit 12 is limited. In normal apparatus, continuous working is required such as at least plural days, if possible, plural years. In short, a technical problem how the cold reserved time(adiabatic time) is prolonged is still remained.